Optical Path Difference (OPD) Formula

Equation for calculate optical path difference (opd) is,

OPD = [d₁×n₁] - [d₂×n₂]

where,
d₁ & d₂ are the distances of the ray passing through medium,
n₁ - is the greater refractive index (eg: glass),
n₂ - is the smaller refractive index (eg: air),
OPD - optical path difference.